Proof? Consider all the objects that have served as money throughout history. Try taking any of them to the aforementioned grocery store. Select something. Get in line. Attempt to pay. See what happens.
In the alternative, use a non-local currency like yen or Euros. Imagine the consequences. The simple truth: If you and the grocer do not agree about the money, no matter how much energy it represents, no business takes place.
So it goes with all manner of money. It requires its collaborators to see eye to eye about what it is, how it works and what a fair trade seems to each under the circumstances.
Why do folks have such trouble with this question? I will suggest a couple of reasons and would be delighted to hear more. For one, money talk per se remains a 21st century taboo. We simply do not talk or think much about it in thoughtful manners.
Then, there is the "fish in water" metaphor. As fish do not notice water or have conscious regard for its nature, modern humanity functions constantly within money and its generated money forces. Money is so engrossing as to be literally unremarkable. We are observably oblivious to it and its impact upon us.
Obviously, for many, perhaps most, much of this agreement comes unconsciously. We grow up amidst money. We assume its existence and its miscellaneous qualities. Yet, at root, money emanates from our collective consciousness. Yet, if a bunch of us agree to particular commoditized vehicles of reciprocity and exchange, we have money-at least between us.
Though this seems simple enough, the implications are huge. It means we humans are in control, not money. For example, it means we can choose to direct the use of the money principles to create designer currencies. It means we can harness money's self-organizing powers.
Money can be something that encourages certain forms of behavior or contains layered considerations and incentives. Practically speaking, "money" can range from frequent flier miles to community currencies such as Ithaca Time Dollars to LETS bartering networks and so on in infinite arrays and variations. There are unique currencies serving as large-scale bartering systems and specialized currencies for localized, targeted time-based systems. Though still at their beginnings, there are specialized designed currencies serving such targeted functions and purposes as education, neighborhood cleanliness, buddy systems, care for the elderly and the infirm, alternative justice systems and international monetary stability. These are among our collective choices.
Remember that money is purely symbolic within our agreement. It is what we say it is. So long as we have our agreement, we can access the human energies and resources to do whatever we want to do. Money of all sorts is about unmet needs and unused energies. Another Bernard Lietaer enlightening observation, "It makes no more sense to say 'we don't have the money to do this' than to claim we cannot build a house because we do not have the inches."
Our money agreement is our deal. It is our arrangement. We are in control whether we know it or not.